The present invention relates to a silicone rubber composition or, more particularly, to a silicone rubber composition capable of being cured by hot-air vulcanization under normal pressure to give a cured silicone rubber having no problems in respect of voids and surface tackiness as well as adverse influences against human body and useful as a material of extrusion molding to prepare highly heat-resistant and electrically insulating electric wire insulation along with transparency. The invention also relates to a cured silicone rubber article which is a vulcanizate of the above mentioned novel silicone rubber composition.
As is well known, silicone rubbers in general have excellent heat and cold resistance and electric properties so that they are widely used in a variety of applications. Silicone rubbers are shaped and cured into cured rubber products by several different processes depending on the type of the rubber composition and the characteristics required in the cured products.
Among several methods for curing silicone rubber compositions, the most conventional is that a silicone rubber composition admixed with an organic peroxide is heated at an elevated temperature. Various organic peroxides have been proposed and practically used as a curing agent of silicone rubber compositions including, for example, benzoyl peroxide, 4-chlorobenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane, tert-butyl perbenzoate, tert-butyl cumyl peroxide and the like. The curing process of a silicone rubber composition formulated with these organic peroxides must be performed under pressure in order to avoid occurrence of voids in the cured rubber body or blistering on the surface.
It is sometimes desirable that curing of a silicone rubber composition can be performed under normal pressure in a hot-air oven as in the fabrication of extrusion-molded rubber articles such as tubes, sheets, insulation of electric wires and the like. None of the above named organic peroxides is suitable as a curing agent in such a process of hot-air vulcanization of silicone rubber compositions excepting 2,4-dichlorobenzoyl peroxide. This particular organic peroxide as a chlorine-containing compound, however, has disadvantages of corrosiveness against metals and adverse influence against human body by the decomposition products thereof. Moreover, cured silicone rubber products obtained by use of this organic peroxide are also not satisfactory in respect of the relatively low heat resistance under a hermetically sealed condition and large permanent compression set.
U.S. Pat. No. 4,061,704 discloses several tert-alkylperoxy alkyl carbonates which can be used as a curing agent of a silicone rubber composition without the above mentioned problems and disadvantages including tert-butylperoxy octadecyl carbonate, tert-amylperoxy-2,6,8-trimethyl-4-nonyl carbonate, tert-amylperoxy octadecyl carbonate and the like. The activity of these carbonate-type peroxides as a curing agent of silicone rubbers, however, is relatively low so that full curing of a silicone rubber composition can hardly be obtained by using the peroxide of this type alone in addition to a problem that the decomposition products of these peroxides are compounds having a relatively large molecular weight and can hardly be dissipated from the cured silicone rubber products to badly affect the heat resistance of the rubber products.
An alternative type of silicone rubber compositions suitable for hot-air vulcanization under normal pressure includes the so-called addition reaction-type silicone rubber compositions which utilize the hydrosilation reaction taking place between an organopolysiloxane having aliphatically unsaturated groups such as vinyl and an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms in the presence of a platinum catalyst. As compared to the peroxidecurable silicone rubber compositions, the applicability of the silicone rubber compositions of this type is limited to several special fields wtihout versatility because of the short pot life or shelf life readily to cause scorching.
Further, several organic peroxides have recently been proposed as a curing agent of silicone rubber compositions including bis(2-methylbenzoyl) peroxide disclosed in Japanese Patent Kokai No. 59-18758, bis(dimethylbenzoyl) peroxides disclosed in Japanese Patent Kokai No. 60-16968, bis(trimethylbenzoyl) peroxides disclosed in Japanese Patent Kokai No. 60-163860 and bis(2-methoxybenzoyl) peroxide disclosed in Japanese Patent Kokai No. 59-232145. These bis(substituted benzoyl) peroxides are indeed effective to solve the problems due to blooming of the decomposition products on the surface of the cured rubber products and the problems in safety and hygiene. A problem in the use of these peroxides is the relatively strong surface tackiness of the silicone rubber products cured by the hot-air vulcanization using the same. Moreover, silicone rubber products cured by using these peroxides can be imparted with an absorbance of ultraviolet light at a wavelength of 220 nm decreased to 0.3 or smaller in the ultraviolet transmission test undertaken for medical implement only when the hot-air vulcanization is followed by a lengthy post-curing treatment.